Question:
I know this should be obvious, but it's not obvious to me: how do I save an InDesign document down to a lower version? I'm using InDesign CS3, but my client uses InDesign CS2.

Answer:
You're right -- it's not at all obvious how to do that. It's one of those things that is completely baffling until you do it the first time, and then it's easy. The key is to export the document into Adobe's InDesign Interchange format. Just choose File> Export and then in the Format field, choose InDesign Interchange. The name of the resulting file will have a .inx extension. This .inx file can then be opened in InDesign CS2.

However, there are a couple of things to keep in mind. First, any features new to CS3 will be ignored in CS2. Second, your client will need to update to the latest revision of CS2. Just after Adobe releases a new version of InDesign, they release an updater for the previous version so that it can read the .inx file of the newer version.

Here's a bit more about that .inx format: it's actually a straightforward XML file that can be edited with a text editor -- although I don't recommend doing that. One benefit of this is that the file is much smaller than the native InDesign document because it doesn't include any image previews or other InDesign-specific goodies. Another benefit is that it can "clean up" an InDesign document by removing any data left behind when, for example, page items were deleted. So, you can use this .inx trick to fix a misbehaving InDesign document: just export it in .inx format and then open the .inx file in InDesign. InDesign will rebuild the document from scratch, instead of relying on the older data in the original InDesign document.